Directly repeated, 20-bp sequence of plasmid R1162 DNA is required for replication, expression of incompatibility,...

DNA required in cis for the replication of the broad-host-range plasmid R1162 is located on two contiguous HpaII fragments of 210 and 370 bp. The latter of these contains three and one-half, perfectly conserved, 20-bp directly repeated sequences. The significance of these for plasmid replication, incompatibility, and copy-number control was examined by generating deletions into these repeats and testing the properties of the remaining DNA. We conclude from the results that the direct repeats are essential for expression of incompatibility and for the decrease in copy number observed when the directly repeated DNA is cloned into R1162. Little, if any, additional DNA is required from the ori region for these properties. Moreover, deletions of intermediate size result in an intermediate level of incompatibility, indicating the importance of the periodic structure of the direct repeats. The directly repeated DNA is also required for an active origin of replication, as are additional, nonrepeated sequences adjacent to this DNA. The properties of the direct repeats are discussed with respect to their possible role in the replication of R1162 DNA.